If you lock it in the same place for school/work then get a big nice heavy chain/U and leave them to the rack where you park your bike. For the rest of the time a folding lock should work:
Try avoiding cable locks
This look interesting I tried find a folding lock but the $80 price seemed very high. ttp://clevercycles.com/p/?prod-code=ABUS+Bordo+6100%2F75+folding+lock%2C+combination#ABUS-Bordo-6100-75-folding-lock--combination